SPV: Plumb through XFB buffer and stride information.

Also, only emit this XFB information where the SPIR-V spec says
it should be emitted: essentially, on objects.

This and the previous commit together fix #1185.

gtests/Spv.FromFile.cpp

@@ -329,6 +329,7 @@ INSTANTIATE_TEST_CASE_P(
         "spv.storageBuffer.vert",
         "spv.precise.tese",
         "spv.precise.tesc",
+        "spv.xfb.vert",
     })),
     FileNameAsCustomTestSuffix
 );